1. Field of the Invention
The present invention relates to an illuminance sensor of a voltage output type.
2. Description of the Related Art
A conventional illuminance sensor is described. FIG. 5 is a circuit diagram illustrating the conventional illuminance sensor.
Based on incident light, photodiodes PD1 and PD2 respectively cause photocurrents Ipd1 and Ipd2 to flow. A differential current between the photocurrent Ipd1, which is amplified by a current mirror circuit including NPN bipolar transistors 51 and 52, and the photocurrent Ipd2 is derived from a collector of a PNP bipolar transistor 53. This differential current is amplified by a current mirror circuit including PNP bipolar transistors 53 and 54, and the amplified current flows to the PNP bipolar transistor 54 as an output current Iout, which indicates the illuminance level of the incident light. The output current Iout is converted into a voltage by a resistor (not shown) (for example, see Japanese Patent Application Laid-open No. 2006-148014 (FIG. 4)).
However, in the conventional technology, when the illuminance of the incident light increases, the photocurrents Ipd1 and Ipd2 accordingly increase, and hence the output current Iout also increases. As a result, a consumption current of the illuminance sensor increases. In other words, the consumption current of the illuminance sensor depends on the illuminance level of the incident light.